<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 200px;
      height: 200px;
      background-color: pink;
    }

    .active {
      width: 200px;
      height: 200px;
      color: red;
      background-color: black;
    }
  </style>
</head>

<body>
  <div class="box">li</div>
  <script>
    const box = document.querySelector('.box')
    //style
    // div.style.width = '100px'
    //className
    box.className = 'box active'
    //classList
    // box.classList.add('active')
  </script>
</body>

</html>